* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}


html,
body,
#app {
  width: 100%;
  height: 100%;
}


header, footer {
  height: 46px;
  display: flex;
  justify-content: space-around;
  align-items: center;
}

footer .active {
  color: red;
}

main {
  flex: 1;
}

main>div{
  display: none;
}

main>div.active {
  display: block;
}

#app {
  display: flex;
  flex-direction: column;
}